I'll do them both from the chin bar as well as on the dip bars. I like the chinning bar becasue it allows you to contract the lower lats in their utmost stretched position. Keep in mind that this requires a tremendous amount of weight to be effective, just as regular shrugs do for the traps.
One word of caution. When doing these from above, be careful not to over stretch the shoulder joint. If you are not careful you will get a bit of subluxation...which isn't good. Too much of this will cause instability of the shoulder joint.